Mnchvgs79

New wheels and brand new sensors and my right rear wheel reads incorrect pressure and shows the pressure drops a couple PSI per day. I'd just assume it was a bad sensor but it did the same thing with the stock wheels in the same position, right rear. Thoughts? It's annoying because the warning light comes on and it keeps making me stop and check my tire pressure. I'm running 45 PSI and it currently shows 27 PSI. The car is new to me as of only 2 weeks ago.

DarthVader2

When you say you're running 45 PSI. Are you measuring that with a high quality mechanical dial external gauge? And when your car is reporting 27 PSI. if you check the pressure with the external gauge, does it still shows 45 PSI ?

Mnchvgs79

Yes that's read with a digital gauge off the Mac Tools truck. All 4 tires 45 PSI and holding.

DarthVader2

I would try a different external gauge preferably a mechanical dial one. I just don't trust those digital gauges sometime. I think it would be easier to get another gauge rather than going through and replacing the TPMS again.

Mnchvgs79

It's absolutely 100% not the gauge. Like I said... The decreasing pressure on the readout has caused me to pull into a gas station to check pressure, I checked at my dads house with an old style also. 45 psi each and every time. It can't be the sensor either... Unless I just happened to have 2 bad sensors in the exact same position. I have the same issue with the stock wheels installed.

DarthVader2

There's only 3 suspects, 4 if you count the Schrader valve. The wheel, sensor and tire. You tried different wheel and sensor. I presume you're 100% sure your tire don't have a leak. If your gauge indicate 45 PSI exactly on all 4 tires then just ignore the alert for a while and see how low the pressure on your dash will go to.

Mike5215

Aren't there two components to the TPMS at the wheel...the TPMS device in the valve stem assembly, which is a little radio transmitter and the sensors on the car to read the output from the transmitters? Or is there one central sensor for all 4 wheels? I believe there's also a way to tell the car to accept as "normal" whatever the settings are at that moment. Not ideal but it would stop the error messages.

Mnchvgs79

konigstiger

One control unit (10 in diagram) receives signal from all wheel sensors (110) rear monitoring incorporates an antenna (50). If you purchased from MB dealer request they check for fault codes. If not, have an independent check using MB diagnosis system.

DarthVader2

I'm not doubting that your car is showing low tire pressure. I'm more afraid that's it's telling you the truth. Those TPMS sensors are usually pretty accurate with their readings from my experience with 6 previous cars I've owned with them equipped. Granted, they weren't all MB. That's not to say there wouldn't be a first time that they're wrong.

You said that when you check the other 3 tires, your MAC pressure gauge always shows 45 PSI each and every time. However, your car is indicating 41 for FL, 43 for FR and 42 for RL. Are those other 3 sensors misreporting their readings also? These sensors have a 1 PSI resolution and every PSI counts. I'm assuming you didn't round up the reading and they were actual?

Mnchvgs79

Holy crap man. I've used 3 different gauges, It's not the tire pressure. I know you're just trying to help but I'm not an idiot.

Mike5215

Can you swap that wheel to another corner and see if the misreport goes with it?

Mnchvgs79

I've done one better... Swapped the entire set of wheels. It does the same thing with the factory wheels installed.

konigstiger

...And manual reset has no effect, correct.

Mnchvgs79

I can save the tire pressure setting and not get a warning.

Mnchvgs79

AH HA!!!! Figured it out. The TPMS thinks the left is the right and vice versa. The LEFT rear tire was low... Now they all read correctly. The factory wheels must've just been coincidence.

Mnchvgs79

I guess I have 2 leaky tires that need to be fixed. At least now I know.

DarthVader2

Congratulations! I'm glad you got it resolved!

I'm just trying to understand this. So the TPMS was reporting the correct pressure but for some reason the Mac guage kept showing 45 PSI for all the wheels?

Mnchvgs79

45 PSI is me rounding off. I didn't think I needed to say LF 43.5 PSI, RF 44.5 PSI etc...

DarthVader2

Ok, figured that.

This is the part I'm trying to understand. If you were reading 45 PSI on all 4 tires with your gauge everytime, what sensor or other gauge finally led you to believe that one of the tire was not 45 PSI?

Mnchvgs79

I didn't check the pressure (with the gauge) of the all the tires that were reading correctly? I assumed the TPMS was reading correctly except for the tire reading incorrectly.

allenjdmb

LR, even after a new sensor, shows 22 lbs. All orhers are accurate. I assume it needs to be the sds box to check, yes?